An at-will state is one where an employee's employment can be terminated by either party at any time, for any reason, unless the reason is unlawful. Being in an at-will state therefore does not give you a right to take legal action where the company enforces a policy against you, but not others.
Differing treatment may, however, suggest unlawful discrimination. If the reason the policy was applied to you but not others was because of your age, gender, race, creed, religion or other protect class status, you may have grounds to pursue legal action.
In addition to what's already been answered, note that under Florida's established rule for termination of at-will employment, 'where the term of employment is discretionary with either party or indefinite, then either party for any reason may terminate it at any time and no action may be maintained for breach of the employment contract. Although some jurisdictions recognize exceptions to this rule, Florida law provides no action for the common law tort of wrongful termination. Florida also has no exception where termination is founded on an employee's exercise of constitutional rights. Thus, to bring a non-contract claim for wrongful discharge in Florida, an employee must rely on statutory causes of action created by the Legislature. So, if what got you fired was not considered protected activity, then your chances of success in a lawsuit will not be great.